The present invention relates to current mirrors, and more particularly to programmable current mirrors.
Current mirrors are used widely in analog and electric circuit design. Simple current mirrors are configured to produce an output current (IO) that is a fixed ratio of a reference current (IR). The reference current is received by a reference transistor having an associated width-to-length ratio (WR). The gate of the reference transistor is connected to the gate of a mirror transistor having a width-to-length ratio (WM). The applied reference current determines the gate voltage arising at the reference transistor, which in turn is passed to the gate of the mirror transistor. The gate voltage on the mirror transistor in turn determines the magnitude of the output current drawn by the mirror transistor. In such an arrangement, the reference current and output current are related by the following:       I    0    =                    W        M                    W        R              xc3x97                  I        R            .      
In general, the factor by which the reference current is multiplied (i.e., WM/WR) may be referred to as the mirroring parameter of the current mirror. Selecting a reference transistor and a mirror transistor with specific width-to-length ratios sets the mirroring parameter and thus the relationship between the reference and output current.
Post-design control over the mirroring parameter is possible through use of programmable current mirrors, which provide programmable variation of the ratio between the output current and the reference current. Typically, instead of a single transistor on the mirror (output) side, a programmable current mirror utilizes a linear array of mirror transistors wired in parallel and connected gate-to-gate. A switching mechanism is associated with one or more of the mirror transistors, in order to selectively activate and deactivate the transistor, and thereby control whether current is permitted to flow through the transistor. In such configurations, WM in the above equation becomes the sum of the individual width-to-length ratio(s) of the activated mirror transistors, such that operation of the switching mechanism allows for some degree of control over the mirroring parameter. Because the mirror transistors are coupled in parallel, the output current is the sum of the currents flowing through the individual activated mirror transistors.
It typically is desirable for a programmable current mirror design to provide a large range of operation and high resolution. A current mirror having a large range of operation is capable of a large range of programmable variation over the ratio of the output current to the reference current; while a current mirror with a high resolution is capable of making such variations in many small steps. Designing programmable current mirrors with high resolution and a large operational range while keeping the physical realization of the device small has proven to be particularly difficult. In order to increase resolution, previous designs have sacrificed range, while attempts to increase range have led to sacrifices in resolution. In many cases, attempts to improve performance in either or both of these areas has undesirably increased the physical realization of the circuit in existing current mirrors.
A programmable current mirror is provided. According to one aspect of the invention, the programmable current mirror includes a reference system configured to receive a reference current and a mirror system operatively connected to the reference system and configured to output an output current that is based on the reference current. The relationship between the reference current and the output current is described by a mirroring parameter. The reference system includes a plurality of transistor groups, each transistor group being configured to alter the mirroring parameter via programmable variation of a dimensional parameter associated with the transistor group. Variations to the mirroring parameter produced by one of the transistor groups are scaled relative to variations produced by another of the transistor groups. According to another aspect of the invention, the reference system and/or mirror system is provided with a multistage configuration. According to yet another aspect of the invention, the programmable current mirror is configured to provide inverse linear programmability.